high pitched whining noise after running A/C

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I own a 1994 Oldsmobile 88 Royale. Last summer it started making a high pitched whining noise that alternated frequency whenever I ran the air conditioner. I stopped using the a/c, but now it sometimes makes that noise on a very hot day (mid 90's), after running for about 20 minutes. I have not used my air conditioner in months because of the noise. We replaced the serpentine belt and that did not fix the problem. Sitting inside the car, it sounds like the noise is coming from the front passenger side. Outside the car, everyone thinks the noise is coming from the back/fuel tank area. The car has had a new fuel pump and filters recently as well. Someone on here mentioned the A/C Compressor, so we had that replaced. The system was properly flushed out and replaced with new fluids. My car is still making the whining noise when I run the A/C. What could be the problem?